The present invention generally relates to a developing apparatus, and more particularly, to a developing apparatus for developing an electrostatic latent image formed on a recording medium as applied, for example, to an electrostatic transfer type copying machine, laser printer or the like.
Commonly, in a copying machine or the like, for the purpose of visualizing an electrostatic latent image formed on a recording medium such as a photosensitive member, into a visible image, there is provided a developing apparatus for causing toner composed of a colored pigment to adhere onto the electrostatic latent image. Particular, in some copying machines, there are provided a plurality of developing units respectively containing different colors of toner for developing the electrostatic latent image in various colors as well as in black and white by selectively using the developing units as required. By way of example, for obtaining a full colored image, developing units respectively containing toners in cyan, magenta, and yellow and further, in black depending on necessity are provided, and by developing the electrostatic latent image on the recording medium subjected to color separation through filters of respective colors with toners in various colors, such toners are overlapped on a transfer paper sheet, thereby providing a copied image in the full color.
The developing apparatus or developing unit as referred to above, is required to be controlled so that the toner does not adhere to the electrostatic latent image when the developing apparatus is not used for the development, and accordingly, it may be so arranged that a developing gap (i.e. a distance between the recording medium and the developing apparatus) is changed over so as to prevent the toner from contacting or being transported to a developing region confronting the recording medium.
However, when the developing gap is changed over in the case where the developing apparatus is not used for the development as described above, scattering takes place in the developing conditions, thus resulting in variations in the image density. In particular, if the positioning of the developing gap is not accurately determined, it becomes impossible to form normal images at all times.
Moreover, in the case where a magnetic brush developing system is employed for the developing apparatus, it is controlled, in some arrangements, as to prevent the magnetic brush from contacting the recording medium by deviating positions of the magnets constituting the developing region, but by such deviation of the magnet positions, there are cases where not only the image density is varied, but the image quality is markedly deteriorated due to adhesion of carrier particles (when a two or dual-component developing material is used) to the recording medium.